1. Field of the Invention
The present invention relates to a method of producing ferroelectric ceramics of perovskite-type crystal structures which are represented by the general formula of ABO.sub.3, and particularly to a method of making transparent ceramics represented by the general formula of ABO.sub.3 wherein A includes Pb and at least one element selected from the group consisting of Ba and Sr and B is a single substance of Zr, both of Zr and Ti or both of Hf and Ti.
2. Brief Description of the Prior Art
Transparent ferroelectric ceramics are exceedingly useful materials for various kinds of light moculators and for image storage and display device since they have superior electro-optic properties.
There have heretofore been known in the art ferroelectric ceramics such as barium titanate (BaTiO.sub.3) and a solid solution of lead titanate and lead zirconate (PbTiO.sub.3 -PbZrO.sub.3), which are practically used as a dielectric material, a material for condensers and a piezoelectric material. However, these known ceramic materials are scarcely pervious to light, so that the uses thereof are limited in the aforementioned fields and the applications thereof for the electro-optic field have not been realized at all. In general, most of the materials having crystal structures of perovskite-type represented by the general formula of ABO.sub.3 exhibit ferroelectricities. There have been known as the A ion of the above formula Pb and Ba, and as the B ion Zr and Ti. However, any of the materials composed of the aforementioned known ions are not pervious to light.
Ceramics are, as different from single crystals, composed of a large number of fine crystal grains. Ferroelectric ceramics have the same constructions wherein the crystal axes of individual crystal grains are in substantially random orientations. For this reason, when the crystal grain has an optical anisotropy a disconuous shift of the refractive index is caused at the interface of each crystal grain, i.e. at the grain boundary, due to the optical anisotropy.
It will be described hereinbelow the circumstances when a light passes through a ceramic. There has been known that if a light is allowed to pass through a medium including a portion or portions at which the refractive index is discontinuously shifted, the light is always refracted or reflected at such the portion or portions. In the ceramics composed of crystal grains having optically uniaxial or biaxial anisotropies, the grain boundaris and the ferroelectric domain walls act as scattering centers of the light. On the contrary, in case where a ceramic is composed of optically isotropic crystal grains, the grain boundaries do not act as scattering centers any longer since the refractive index is not discontinuously shifted at the grain boundaries. It has thus been derived the conception that the basic condition for obtaining a transparent ceramic is to find out a ceramic material composed of crystal grains which do not exhibit optical anisotropies.
However, the anisotropic characteristics of the ferroelectric materials are considered as the inherent characteristics thereof, and consequently it is also considered inevitable that the unexceptional anisotropies of the individual crystal grains of the ferroelectric ceramics are the characteristic features of such the ceramics. It is accordingly concluded that transparent ferroelectric ceramics which do not cause any scatterings may not be produced in the strict sense of the word. But it is reasonably expected that a ferroelectric material composed of crystal grains having small degrees of anisotropies causes only a small extent of scattering of the light and is capable of being used as a transparent ferroelectric material.
It is further required for the transparent ceramic to be a material which absorbs an extremely small amount of visual lights. It is generally known that the intensity of the light transmitted through a medium or substance decreases in conformity to the following equation: EQU I .alpha. I.sub.0 e.sup.-.sup..alpha..sup.t
wherein I.sub.0 is an incident light intensity, I is a transmitted light intensity, t is a thickness of the sample substance, and .alpha. is an apparent absorption coefficient of the substance. The apparent absorption coefficient .alpha. in the above equation is calculated from the formula including a term with respect to intrinsic absorption due to the absorption of energy and a correction term with respect to scattering caused at points of discontinuity of the refractive index, voids in the ceramic or the grain boundaries. The intrinsic absorption coefficient is inherently defined corresponding to the kinds and the arrangement of the atoms constituting the substance and represents the absorption of the light energy caused by the transition of electrons in the substance. As a consequence, materials for the transparent ceramics are those which have not the intrinsic absorption in the visual range.
It is, therefore, concluded from the above described facts that the materials for the transparent and ferroelectric ceramics shall be those composed of crystal grains which have anisotropies of small degree and do not absorb the visual lights.
On the other hand, there have recently made vigorous investigations for making a transparent ceramic in the field of oxide system ceramics and many attempts have been made to obtain transparent ceramics made of, for example, aluminium oxide (Al.sub.2 O.sub.3), magnesium oxide (MgO), yttrium oxide (Y.sub.2 O.sub.3), zirconium oxide (ZrO.sub.2), and thorium oxide (ThO.sub.2). Up to the present day, ceramics are considered as optically opaque materials, but recently transparent ceramics may be produced by preventing the formation of voids during sintering.
Ceramics are generally obtained by a method which comprises moulding pulverized powders of starting materials and sintering the moulded powders. A number of voids and hollow channels are inevitably included in a green compact formed by the moulding. The total numbers and/or volume of the voids and the hollow channels vary in accordance with the kind of the powder used and the moulding pressure employed, but it is impossible to completely prevent the formation of voids in the green compact only by varying the condition of the compression moulding and to obtain dense ceramics having teoretical densities. In order to vanish the voids for making dense ceramics, a sintering process is indispensable with and in which process almost all of the voids in the green compacts are vanished. It is now to be noted that the voids in the ceramics which act as scattering centers shall be essentially vanished in order to produce transparent ceramics and that the porosities of the known transparent ceramics are less than about 0.5% and no transparent ceramic has been found with the porosity of more than about 0.5%. It is thus concluded that the transparency of ceramics is affected by the residual voids therein and that the porosity due to the voids of the ceramics is varied in accordance with the conditions of the sintering.
There have been proposed some improvements in the known production method of transparent ceramics with respect to the sintering operation thereof in consideration of the above facts. For example, in a method of producing a transparent alumina there has been proposed a method of obtaining a semitransparent sintered alumina which comprises adding a very small amount of magnesium oxide (MgO) into aluminium oxide (Al.sub.2 O.sub.3), moulding the resultant mixture and sintering the moulded article in a hydrogen atmosphere at a high temperature. The added MgO is segregated at the grain boundaries to prevent the exaggerated grain growth of Al.sub.2 O.sub.3 thereby to facilitate the diffusion for vanishing the residual voids in the crystal grains through the crystal lattice vacancies.
An alternative known method for vanishing the voids in ceramics other than the above described usual sintering method is a so-called hot-pressing method in which the green compact is formed at a high temperature under a compression pressure. In this method, the voids in the sintered body are pressed out under a relatively low applied pressure by utilizing the phenomenon that the resistance to deformation of the powders decreases at high temperature. According to this method the voids in the sintered body may be further decreased to produce a sintered body of high density by employing jointly with the diffusion vanishing method in the usual sintering method. As is apparent from the foregoing description, the principal subject matter to be solved for producing transparent ceramics is to determine the appropriate sintering conditions for vanishing the voids in the ceramics.
There has recently been developed a transparent ferroelectric ceramics called under the name of "PLZT" which scatters lights only at a small extent. This kind of ceramics is the only one which is presently available as transparent ferroelectric ceramics and has a composition of (Pb, La)(Zr, Ti)O.sub.3. This kind of ceramics is produced by adding 5 - 10 at % of La to Pb(Zr, Ti)O.sub.3 and a conventionally known piezoelectric ceramic. It has not been theoretically made clear the reasons for the considerable increase in the transparency of the ceramics obtained by dissolving a small amount of La in a form of solid-solution although the Pb(Zr, Ti)O.sub.3 per se is not pervious to light. It has thus been assumed that La is essentially added so as to make a solid solution therewith in order for obtaining a transparent ferroelectric ceramics.
However, the known transparent ferroelectric ceramics of PLZT have a disadvantage in that they are apt to be broken down when an alternating current voltage is applied thereto since the lattice structures of the ceramics of the PLZT series inevitably include therein a number of crystal lattice vacancies induced by the charge compensation occurred by the addition of La and they show considerable electrostrictive effects. In contrast thereto, ceramics produced in accordance with the method of the present invention are not broken down when an alternating current voltage is applied thereto and they show satisfactorily stable domain switchings since they do not substantially include crystal lattice vacancies.